The ULTRAVIOLET RISK INDICATOR (UVRI) is intended to warn people about the damage to human health which may result from exposure to UV radiation of the solar spectrum, depending on the altitude of the sun in the sky and its relationship with the intensity of UV radiation at a particular place and time. UV solar radiation also depends on other factors, such as the stratospheric ozone layer, the presence of clouds, latitude of the place, elevation above sea level, and reflection of solar radiation from the surface of Earth. The instrument basically consists of a dial with a shadow projection device on it (a polar or straight stylus). The polar stylus is parallel to the Earth's axis of rotation. The straight stylus is perpendicular to the dial and shows areas of different colors indicating different degrees of UV risk. The dial is defined by lines of declination (solstices or equinoxes) and solar hour lines calculated on the basis of the transformation of Solar Hour Coordinates (Hour Angle and Declination) in the 3D Cartesian Coordinates System, whose origin is the tip of the stylus (polar or straight), using a matrix calculation to produce rotations of the coordinate system that take into account the latitude of the place, declination (orientation) and inclination of the dial. In general, the style is a rod or triangular shape, whose parameters are calculated, the tip of the style is the projection focus or point of view of a gnomonic projection, where the “point” of space to be projected is the sun. The dial is the plane projection, and the point projected is the “shadow point”. The coordinates of the shadow point (x,y) of an orthogonal coordinate system located in the plane of the dial are calculated.
Apart from the hour lines of the local solar time, which will be concurrent to the so-called center of the dial, the declination lines of the solstices and equinoxes and the location on the dial plane of the shadow projected by the end of the projection device (polar or perpendicular) are calculated, which moves according to the annual and diurnal variation of the altitude of the sun crossing different sectors of the dial, with different colors indicating the degree of risk of UV radiation at the instant of the observation.
There are no known instruments like the UV RISK INDICATOR for the purpose of providing information in real time in situ about the solar UV radiation risk, in a simple form and which is available to any observer, even a child, since normally the operation, reading and interpretation of data from radiometers, spectroradiometers and dosimeter instruments for measuring the intensity of solar or UV radiation is reserved for qualified technicians. There are also organizations in different countries, usually meteorological services that report the UV Index based on measurements with spectroradiometers or broadband detectors. For prediction of a UV Index a radiative transfer model is required with input of total ozone from ground-based spectroradiometers or from satellites, and the aerosol optical properties. According to the country, this type of warning is not always taken into account by people when they are gauging appropriate sun protection, and there is a general lack of knowledge about the critical values of UV radiation.
1.1. Extraterrestrial Radiation
The solar radiation incidence outside the atmosphere of the Earth is the extraterrestrial radiation (FIGS. 1 and 2).Io=1367*(Rm/R)2 W/m2
Io extraterrestrial irradiance (FIG. 2)
Rm mean distance Sun-Earth
R actual distance Sun-Earth
An approximate equation (Rm/R)2 is:(Rm/R)2=1.00011+0.034221*cos+0.001280*sin+0.000719*cos(2*β)+0.000077*sin(2*β)β=2*pi*n/365 radians
n day of the year
The Earth has an elliptical orbit around the sun that is located in one focus of the ellipse (Kepler's 1st Law). The maximum value of extraterrestrial irradiance (1414.9 W/m2) occurs each year on the perihelion, in early January, when the distance Earth-Sun is minimal. The minimum irradiance (1321.3 W/m2) is reached on the aphelion, in early July, when the distance Earth-Sun is at its maximum point. The average value of extraterrestrial irradiance is 1367.1 W/m2.
The highest values of extraterrestrial radiation coincide with the summer in the Southern Hemisphere (winter in the Northern Hemisphere). The lowest values occur when it is winter in the Southern Hemisphere (summer in the Northern Hemisphere).
1.2. Electromagnetic Spectrum
RadiationWavelengthEnergyType(nm)(%)Infrared>70049.4Visible400-70042.3UVA400-3206.3UVB320-2901.5UVC<2900.5Total100.0
The table above shows the distribution of solar irradiance energy. The ultraviolet radiation can be subdivided into UVA, UVB and UVC and is a specific part of the spectrum, which represents only 8.3% of total energy. Most of the radiation is in the visible and infrared zone (FIG. 3).
The UVA radiation includes the wavelength range between 320 and 400 nm and it is absorbed to a small extent by the ozone layer in the stratosphere of the Earth. The UVB radiation, with wavelengths between 290 and 320 nm, is mostly absorbed by the stratospheric ozone. The UVC radiation is characterized by wavelengths shorter than 290 nm and it is completely absorbed by the ozone.
The radiation reaching the Earth is largely composed by UVA and a small amount of UVB radiation. This is the part of the spectrum that may affect human beings if they are exposed for a long time to sunlight.
1.3. UV Index
The UV Index is a daily forecast of the amount of skin damaging UV radiation which is expected to reach the surface of the Earth when the altitude of the Sun at its maximum point in the sky.
The amount of UV radiation reaching surface of the Earth is mainly influenced by the altitude of the Sun above the horizon of a place, the ozone levels in the stratosphere and the cloud cover. The UV Index ranges from 0 at night hours to 15 or 16 on the tropics and high places under clear skies.
1.4. Diurnal Variability of UV Radiation
FIG. 4 shows Diurnal Variation of Ultraviolet Radiation Erithemal Dose Rate mW/m2 (ordinates), Local Solar Time (abscissas)
The UV radiation and its likelihood to affect skin are highest when the Sun reaches its maximum altitude in the sky or solar noon (zenith distance z=0), and it decreases rapidly as the Sun approaches the horizon (FIG. 4).
FIG. 5 shows the diurnal variation of UV solar radiation and its likelihood to affect skin for different latitudes in Northern Hemisphere and summer solstice. The UV radiation presents strong variations between 08:00 and 16:00 approximately of local solar time for latitudes 20° N and 40° N, and an attenuated variation for latitude 60° N, with a maximum value at solar noon much higher for the first two. The hour angle ±4 h (±60°) of the Sun corresponds to 16:00 and 08:00 h of true solar time, westward and eastward respectively measured from transit or solar noon.
1.5. UV Radiation and the Altitude of Sun
The amount of ozone crossed by solar radiation depends on the concentration in the atmosphere, the altitude above sea level and the angle of the Sun over the horizon of a place.
The higher the altitude above sea level (mountain) the shorter the path of the radiation in the atmosphere, which therefore results in an increase in the irradiance.
If the sun is low over the horizon (FIG. 6), that is, if there is a large zenith distance, the greater is the path of solar radiation in the atmosphere and the amount of stratospheric ozone that must be crossed is longer, therefore the level of irradiance at this point on the surface of the Earth is lower.
If zenith distance z=0 the Sun is directly above the place (zenith), that situation is only possible for latitudes between 23.44° N and 23.44° S, Tropics of Cancer and Capricorn respectively.